1. Field of the Invention:
The invention in general relates to a method of making pressed anodes for electrolytic capacitors and more particularly to a binder material which results in highly porous and strong anodes which yield capacitors with low d.c. leakage.
2. Description of the Prior Art:
Pressed anode bodies for electrolytic capacitors are generally formed by pressing a powder of film-forming metal, which is usually tantalum. ("Film-forming" means that the metal is capable of forming an oxide film on its surface which acts as a capacitor dielectric.) The anode body is then sintered to form the anode, which is then anodized to add an insulating layer. The finished capacitor is then formed by adding a cathode material and enclosing the anode in a suitable case.
It is known to add a binder to the tantalum prior to pressing. The binder assists in holding the metal particles together and in forming and maintaining pores in the metal as well as reducing frictional wear of the dies and punches. The binder is removed from the pressed body prior to sintering. Binders are chosen for three basic properties: lubricity, adhesiveness and ability in forming pores. The lubricity assists the metal particles in sliding over one another during pressing to form a full, well-formed anode body; the adhesiveness assists in holding the particles together, and the pores lead to higher capacitance and lower E.S.R. in the finished capacitor. The field of binders has been a major research area in the art since a substance that is high with respect to one of the three properties is often low with respect to the others. The binder is usually added to the metal powder in powder form or dissolved in a liquid, with the binder coating the metal powder upon evaporation of the liquid solvent. The binder must be removed after pressing and before sintering, otherwise it will react with the metal and interfere with its film-forming property and result in inferior capacitors with high d.c. leakage.
Polyethylene oxide has been used as a binder in making anodes for solid electrolytic capacitors for many years. With the advent of higher surface area tantalum powders, the removal of polyethylene oxide and other binders from the pressed anode bodies has become increasingly more difficult. When the surface area of the tantalum powder exceeds the range of approximately 8000-10,000 CV/gm, the removal of traditional binder materials such as polyethylene oxide becomes uneconomical due to the quality of the vacuum required in a binder removal furnace (i.e. the high vacuum required cannot be maintained in a cost-effective fashion due to the volume of binder material which much be removed). If higher temperatures are employed in order to drive off the binder an undesirable reaction occurs between the tantalum and the binder which gives rise to poor electrical properties in the finished capacitors (i.e. high d.c. leakage). The maximum temperature range to which tantalum may be heated in contact with binder materials without reaction is 250.degree.-300.degree. C. although anodes are frequently heated considerably hotter once the binder has been removed in order to increase their strength prior to actual sintering.
In order to avoid the high temperature reaction between tantalum anodes and contained binder, volatile materials such a camphor have been used successfully with high surface area tantalum powders, but the use of materials such as camphor results in mechanically weak anodes which are susceptible to crumbling due to rapid evaporation of the binder at room temperature.